Image-guided radiation therapy (IGRT) is a form of radiation therapy that comprises frequent three-dimensional imaging while the radiation treatment is under way. Concurrent images are compared with imaging done before treatment to map targets and organs within the patient and plan radiation therapy. IGRT relies directly on the imaging modalities from planning as the reference coordinates for localizing the patient. For example, IGRT can combine localization of a real-time cone-beam computed tomography dataset with the computed tomography (CT) dataset used in planning.
On-line adjustments to patient and beam position are typically made during the treatment process, based on continuously updated information throughout the procedure. For example, gold markers may be implanted around the tumor to provide a surrogate position of the tumor. Prior to each day's treatment, imaging system results are returned. If the center of the mass has moved greater than a pre-defined tolerance (for example, 3 mm), then the couch is readjusted and a subsequent reference image is created.
For further information, the reader is referred to the following publications. Cossmann, PH, Eur. Oncol Rev. (2005) provides a general review of advances in IGRT. Jaffray, D A, et al. showed how X-ray imaging could be used for verification and localization in radiation therapy. Modern Technology of Radiation Oncology. Madison, Wis.: Medical Physics Pub (1999). Dawson, L A et al. Lancet Oncol. 7:848-858 (2006) discusses the rationale, benefits, and limitations of IGRT.